Mapping and surveying · · Updated on
Survey coordinates: UTM, ETRS89 and geographic
What UTM and geographic coordinates are, what zone 30 and ETRS89 mean, how to convert between them and when the Cadastre asks for them. With an example near Xàtiva.
Survey coordinates are the numbers that fix where a point on a property lies. Spain uses two families, geographic coordinates (latitude and longitude) and UTM coordinates (metres on a flat grid), and the official ones are referred to the ETRS89 system. Here we explain what each one is, how to convert from one to the other and when the Cadastre or the Land Registry will ask for them.
What is a survey coordinate
A survey coordinate is the position of a point expressed in a specific reference system. The numbers on their own mean nothing: 715,320 and 4,316,780 in ETRS89 UTM zone 30 point to one place, and the same figures in the old ED50 point to another, about a hundred metres away. That is why a complete coordinate always carries five pieces of information:
- The geodetic system (ETRS89, ED50, WGS84).
- The type: geographic or projected (UTM).
- The zone, if it is UTM.
- The units: degrees or metres.
- The height reference, if there is a height: above the ellipsoid or above sea level.
A plan that does not state its system cannot be safely overlaid on another one, and that is where many cases of “the Cadastre looks shifted” come from.
Geographic coordinates: latitude and longitude
Geographic coordinates place a point on the Earth with two angles. Latitude measures north or south of the equator and longitude measures east or west of a prime meridian, the one through Greenwich. Xàtiva lies at about 39° N and 0.5° W.
They are written in two ways, and it is best not to mix them:
- In decimal degrees:
38.97355N and-0.51448(the negative sign means west). - In degrees, minutes and seconds: 38° 58’ 24.8” N and 0° 30’ 52.1” W.
They are the ones shown by your phone and by web viewers. For measuring a plot they are awkward: a degree of longitude is not the same length in Xàtiva as in Oslo, and calculating an area from angles means using spherical trigonometry. That is why, for plots, they are converted to a flat grid.
UTM coordinates: metres on a flat grid
The UTM system (Universal Transverse Mercator) divides the world into 60 zones of 6° of longitude and projects each one onto a plane. Within a zone, every point has two values in metres:
- X or Easting (E): distance to the east, counted from the central meridian of the zone, which is given the value 500,000 so that there are no negative numbers.
- Y or Northing (N): distance north of the equator.
With metres, a distance is a subtraction and an area is a polygon formula. That is why almost all plot plans, the DWG files an architect receives and GML files are in UTM.
What zone 30 means
Zone 30 runs from 6° W to 0°, with its central meridian at 3° W. It covers most of the Iberian Peninsula, including the Valencian Community, Murcia and Albacete. Western Galicia falls in zone 29 and the Canary Islands in zone 28.
What matters is that the plan always states the zone it was made in. On ours it is written out: ETRS89 UTM 30N.
UTM is not the ground
The projection distorts distances slightly, and the distortion grows with distance from the central meridian. Around Xàtiva, about 2.5° from the central meridian, 100 m measured on the ground come out as 100.017 m on the UTM grid, a difference of about 1.7 cm. It is small, but it explains why two programs can give areas that differ in the last digit. Heights above sea level are not the ellipsoid heights either: on the Valencian coast they differ by about 50 m, the undulation of the geoid.
ETRS89: the official reference system
ETRS89 (European Terrestrial Reference System 1989) is fixed to the Eurasian plate, so the coordinates of a point in the peninsula hardly change over time. It is based on the GRS80 ellipsoid. Royal Decree 1071/2007 established it as the official geodetic system in Spain and gave until 1 January 2015 for official mapping to move over to it.
Before that, ED50 (European Datum 1950) was used. The same point has different coordinates in ED50 and in ETRS89, with a difference of the order of a hundred metres in Spain, so mixing plans from the two draws the plot in the wrong place. Some old urbanisation plans, projects and deeds are still in ED50. We cover the change from one to the other in mapping in Spain: from ED50 to ETRS89.
WGS84, used by consumer GPS, matches ETRS89 at travel-map level. For a plot plan it no longer does: because of plate movement the two can now be up to about a metre apart. That is why precision surveys are calculated and delivered in ETRS89, not from the raw reading of a phone.
EPSG codes identify each system in CAD and GIS programs, and these are worth knowing:
| System | EPSG code |
|---|---|
| ETRS89 geographic | 4258 |
| ETRS89 UTM zone 30 north | 25830 |
| ED50 UTM zone 30 north | 23030 |
| WGS84 geographic | 4326 |
Example: a plot near Xàtiva
Imagine a rural plot with four corners in the municipality of Xàtiva. The coordinates are illustrative: they do not correspond to any real property.
| Corner | Easting (X) | Northing (Y) |
|---|---|---|
| 1 | 715,320.00 | 4,316,780.00 |
| 2 | 715,385.40 | 4,316,775.20 |
| 3 | 715,390.10 | 4,316,708.60 |
| 4 | 715,322.50 | 4,316,712.30 |
The same corners, converted to ETRS89 geographic coordinates:
| Corner | Latitude | Longitude |
|---|---|---|
| 1 | 38.973551° N | 0.514479° W |
| 2 | 38.973492° N | 0.513726° W |
| 3 | 38.972891° N | 0.513693° W |
| 4 | 38.972941° N | 0.514471° W |
System: ETRS89 UTM zone 30 north. With flat coordinates the calculation is a subtraction of columns: side 1-2 is about 65.6 m long and the area on the grid comes to about 4,450 m². Measured on the ellipsoid it would be about 1.5 m² smaller, 0.03%. That is not an error: it is the distortion described above. For that reason, in a Cadastre procedure it is worth stating how the area was calculated.
The same plot in ED50 would have an identical outline, only with different numbers and shifted. If someone gives you “the coordinates of the property” with nothing else, the first job is to find out which system they are in.
How to convert from one system to another
Converting from geographic to UTM, or from ED50 to ETRS89, is a mathematical operation, not a measurement. The steps are the same in any program:
- Identify the source system: datum, type, zone and units.
- Choose the target system: for the Cadastre and the Registry, ETRS89 UTM 30N (EPSG:25830).
- Apply the transformation. From geographic to UTM it is a projection formula. From ED50 to ETRS89 you also need an official transformation grid from the National Geographic Institute (IGN); without it the result can be out by one or several metres.
- Check against a known point: a geodetic benchmark, a building corner shown on the official mapping or a point measured with GNSS.
CAD and GIS programs such as QGIS do the conversion if you give them the codes. The risk is not in the calculation: it is in getting the source system wrong.
Common mistakes
- Swapping X and Y. In Spain the Easting has six digits and the Northing seven. If a program puts the property in the sea, look at that first.
- Forgetting the sign of the longitude. 0.51 instead of -0.51 takes you to the other side of the Greenwich meridian.
- Not stating the zone. UTM without a zone is an incomplete coordinate.
- Using ED50 without knowing it. An old plan overlaid on a current one without transformation comes out shifted.
When the Cadastre and the Registry ask for georeferenced coordinates
A property is georeferenced when its outline is defined with coordinates in the official system, ETRS89 UTM. The Cadastre and the Land Registry ask for that georeferenced graphic representation, usually as a GML file, in procedures such as subdividing or merging a property, correcting its area or boundaries, or registering the geometry of a plot (Law 13/2015 reformed the Mortgage Law for this purpose). Each procedure has its own conditions, and it is the authority or the notary’s office that says when it is required.
If a notary’s office, a lawyer or a formal notice asks you for “the coordinates” or “the GML”, this is what they mean. That file describes the geometry of the property, but it does not prove ownership or replace the procedure of the Cadastre, the Registry or the town council.
What we do at Clotoides with coordinates
We measure the plot with GNSS RTK and a total station, calculate the coordinates of each corner and deliver them in ETRS89 UTM 30N, with the system written on the plan. If the case calls for it, we prepare the GML for the Cadastre and the Registry and a georeferenced plan ready to submit. If you want to know what that file contains and how it is opened, we explain it in the guide to the GML file.
Each job is signed by Vicent Doménech, chartered surveyor no. 8948, from our office in Xàtiva.
Frequently asked questions
Which coordinates does the Cadastre use in the Valencian Community?
The Cadastre works with ETRS89 coordinates in UTM projection, zone 30 in this area. It is the same system in which we prepare the plans and the GML files that are submitted.
How do I know whether an old plan is in ED50?
Normally the title block says so. If it does not and the plan predates 2015, it has to be checked against a known point. If, when you overlay it on current mapping, the whole plan appears shifted about a hundred metres in the same direction, that is a clear sign.
What is the difference between survey coordinates and UTM coordinates?
“Survey coordinates” is the generic name for the coordinates used in surveying. UTM ones are the most common, although there are also local site coordinates, which start from their own origin, and geographic ones.
If you have a property to measure or a plan that does not fit the Cadastre, tell us about it on the contact page or first read about plot and boundary measurement.
Tags georreferenciacionmapaplanotopografia
Written by Vicent Doménech, geodesy and cartography engineer, chartered surveyor no. 8948.
